A number of methods of the transmission and reception of QAM (quadrature amplitude modulation) signals [1] applied in modern telecommunication systems are known. The power losses at demodulation of such QAM signals are defined by:
method of restoration of the carrier frequency,
method of restoration of clock frequency,
selectivity by the adjacent channel,
interference properties or the pulse characteristic of the communication channel
At this the demodulation threshold, i.e. S/N ratio, at which the carrier frequency stops to be extracted is defined by the method of the formation of the signal and the method of the extraction of the carrier from this formed signal.
For modem types of modulation, such as QPSK, the threshold of S/N ratio, at which the carried is extracted amounts to 3-6 dB or in terms of Eb/N0 4.5-5 dB (for the rate of interference stable coding by Viterbi of ¾), and for modulation of the 8PSK type the threshold of demodulation is ever more—about 10 dB. At this in this point the reception is unstable, as the signal can get dropped and then captured again.
In order to reach Shannon threshold demodulation is to be performed already at S/N=4.8 dB for QPSK modulation at the non-coded signal, and at coding the threshold value of S/N to be even less (0 dB for the coding rate of ½, −2.3 dB for rates ⅓ and −3.8 dB for the rate of ¼),
At present systems of interference stable coding, for instance Turbo-coding, which allow to reach Shannon threshold, are known. The hindrance factor is the absence of demodulators, which are capable to work at such low S/N ratios because of the absence of synchronization, which is connected with the methods of the formation of the signal and extraction of carrier from this signal. In the signal spectrums, which use such types of modulation as QPSK, 8PSK, 16QAM and so on there is no remainder of the carrier, therefore its coherent fluctuation is extracted from the received signal by a nonlinear transformation and the subsequent filtration. Such transformation is the method of the multiplication of frequency, which can be implemented by the raising of the input signal into M-degree (into the 4th degree for QPSK, into the 8th degree for 8PSK and so on). But not only the signal but also the noise is raised to the degree, which limits the threshold of the restoration of the carrier At this the phase ambiguity is formed, the removal of which requires the introduction into the signal of relative coding, which introduces the additional power losses.